Thin film sputtering techniques have been used to deposit superconducting materials such as NbN and Nb.sub.3 Ge, as taught by U.S. Pat. No. 3,912,612 (Gavaler et al.) and U.S. Pat. No. 4,043,888 (Gavaler), respectively. More recently, many superconductor films having high critical temperature, T.sub.c, values have been demonstrated, for example (La.sub.0.9 Sr.sub.0.1).sub.2 CuO.sub.4-y with T.sub.c values around 37 K were reported by Yomo et al. in the Japanese Jour. App. Physics, "High Pressure Study and the Critical Current of High T.sub.c Superconductor (La.sub.0.9 Sr.sub.0.1).sub.2 CuO.sub.4-y ", Vol. 26, No. 5, pp. L603-L605 (1987). The material YBa.sub.2 Cu.sub.3 O.sub.7, due to its very high T.sub.c of 92 K, has most recently become a very important superconductor, and its deposition on Y.sub.2 BaCuO.sub.5 in U.S. Pat. No. 4,929,595 (Wu), on ZrO.sub.2 in U.S. Pat. No. 4,959,346 (Mogro-Campero), and its deposition on one of SrTiO.sub.3, LaAlO.sub. 3, MgO, yttria stabilized ZrO.sub.2, or .alpha.-Al.sub.2 O.sub.3 (sapphire) in Gavaler et al., Physica B, 165-166, "Optimization of T.sub.c and J.sub.c in Sputtered YBCO Films", pp 1513 to 1514 (North Holland), 1990, has been reported.
Off-axis sputtering of superconducting multi-layers of YBa.sub.2 Cu.sub.3 O.sub.7 /La.sub.2-x Sr.sub.x CuO.sub.4 have also been very generally reported by Eom et al. in Bull. Amer. Phys. Soc. "Synthesis and Properties of YBa.sub.2 Cu.sub.3 O.sub.7 /La.sub.2-x Sr.sub.x CuO.sub.4 Multilayered Superconducting Thin Films Grown In-Situ by Off Axis Sputtering", Vol. 35, No. 3, p. 383, F17-6 (Abstract) Mar. 13, 1990. A variety of bulk materials have been tried as a substrate material for YBa.sub.2 Cu.sub.3 O.sub.7 films, where YBa.sub.2 Cu.sub.3 O.sub.7 was screen printed on a polycrystalline ceramic sample to produce a film, as reported by Varadaraju et al., in Thin Solid Films, "Superconductivity Behaviour in Screen Printed YBa.sub.2 Cu.sub.3 O.sub.7 Films", Vol. 164, pp 119 to 121, 1988. Alumina substrates were found most suitable, polycrystalline La.sub.2 CuO.sub.4, SrTiO.sub.3 and yttria stabilized zirconia substrates were found deficient, the latter three resulting in T.sub.c zero values of between 77 K to 80 K and either low resistivity or insulating properties.
The best substrates for epitaxial growth of thin films of the high-temperature superconductor YBa.sub.2 Cu.sub.3 O.sub.7 include single crystal LaAlO.sub.3, and MgO. The low r.f. (radio frequency) surface resistance losses and lack of dispersion exhibited by YBa.sub.2 Cu.sub.3 O.sub.7 superconductors make them attractive for use in filters, delay lines, strip-line-resonators, directional couplers, and other passive devices in microwave systems. The (100) orientation of these substrates produces (001) growth of YBa.sub.2 Cu.sub.3 O.sub.7, with, under the correct deposition conditions, high critical temperature and critical current density, and low microwave surface resistance. The disadvantage of these substrates for microwave applications is their high dielectric losses, and in some cases the lack of availability of large substrate sizes.
A more suitable substrate for microwave applications is sapphire (.alpha.-Al.sub.2 O.sub.3), which has very low dielectric losses, and is readily available in large areas. However, the quality of YBa.sub.2 Cu.sub.3 O.sub.7 films grown directly on sapphire is inferior to films on high-loss substrates such as LaAlO.sub.3, and MgO, since the YBa.sub.2 Cu.sub.3 O.sub.7 film interacts with the sapphire at the elevated temperatures needed for formation of the high-T.sub.c phase, leading to a high-loss interface layer and poor film quality.
Talvacchio et al., in Physica C, 162-164, "YBa.sub.2 Cu.sub.3 O.sub.7 Films Grown on Epitaxial MgO Buffer Layers on Sapphire", pp 659 to 660 (North Holland), 1989, taught use of epitaxial MgO as a buffer layer between a YBa.sub.2 Cu.sub.3 O.sub.7 superconducting film and a sapphire substrate in passive microwave devices. A buffer layer of polycrystalline ZrO.sub.2 between a YBa.sub.2 Cu.sub.3 O.sub.7 film and a sapphire substrate was taught by Naito et al., in Jour. Mat. Res., "Thin Film Synthesis of the High-T.sub.c Oxide Superconductor YBa.sub.2 Cu.sub.3 O.sub.7 by Electron Beam Codeposition", 2 (6), pp 713 to 725, (1987). Also, a buffer layer of SrTiO.sub.3 between a YBa.sub.2 Cu.sub.3 O.sub.7 film and a sapphire substrate was taught by Char et al., in Appl Phys. Lett., "Microwave Surface Resistance of Epitaxial YBa.sub.2 Cu.sub.3 O.sub.7 Thin Films On Sapphire", 57 (4), pp 409 to 411 (1990).
The use of MgO, ZrO.sub.2 or SrTiO.sub.3 as buffer layers, however, has still not provided an optimal YBa.sub.2 Cu.sub.3 O.sub.7 combination with a sapphire support for microwave applications, where low rf surface resistance of the superconductor is essential. What is needed is a superconducting deposit on a sapphire substrate which will not exhibit reaction with sapphire and which will exhibit low rf surface resistance. It is one of the main objects of this invention to provide such a composite.